Two-dimensional magnetic recording (TDMR) is a storage architecture that promises to increase the areal density of next-generation hard disk drive products. The ultimate goal in TDMR is for the channel bits to be roughly the same size as the magnetic media grains; in other words, each grain on the magnetic medium ideally stores one bit. In theory, by storing one bit per grain of the magnetic medium, TDMR can achieve a density of 10 terabits per square inch on conventional magnetic storage media available in 2015.
The grains of conventional storage media have nonuniform sizes and shapes, and the bit-size-to-grain-size ratio is lower in a TDMR system than in a conventional magnetic recording system. Thus, reading the information stored by a TDMR system is more challenging than reading information stored by a conventional storage system because the highly irregular grain (and, therefore, bit) boundaries cause noise in the read channel, and the tracks are narrow. A TDMR system can recover the stored bits using two-dimensional signal processing techniques if reasonably high-resolution information is available in the cross-track (i.e., from one track to another) and down-track (i.e., along the track being read) dimensions. To obtain such information, a TDMR system can use an array of two or more read elements to create a two-dimensional array of read-back signals, where one dimension is the cross-track dimension, and the other dimension is the down-track dimension. The read sensors can be aligned in the down-track direction (i.e., over the track being read), or they may be offset so that one or more sensors read data from the desired track, and one or more other sensors sense data in adjacent tracks to account for noise.
To provide good TDMR read head performance, it is desirable for the read sensors to be electrically isolated from each other as well as magnetically shielded from each other. A center shield is typically disposed between each pair of read sensors of a TDMR read head to provide the electrical isolation and magnetic shielding. The center shield is typically stabilized using an exchange-biased antiferromagnetic (AFM) layer deposited on the center shield. The thickness of the AFM layer increases the down-track spacing (DTS) between the read sensors and reduces the areal density capability (ADC) of the TDMR read head. Therefore, a challenge in TDMR systems is to reduce the DTS between the sensors in the TDMR read head.
One way to reduce the DTS is to eliminate the AFM layer altogether, but the unstabilized center shield may induce noise on one or both of the read sensors it is designed to shield and isolate, thereby reducing the TDMR read head performance. Another approach to reduce the DTS is to stabilize the center shield using patterned AFM tabs on each side of the center shield instead of an AFM layer deposited on the center shield; however, to preserve the permeability of the side shield and ensure narrow magnetic read width (MRW) of the lower read sensor (i.e., the sensor under the center shield), these tabs need to be placed away from the lower read sensor. When the tabs are far enough from the lower read sensor to preserve the permeability of the side shield and to ensure narrow MRW, the exchange-biased AFM tabs may not effectively stabilize the center shield.
Therefore, there is an ongoing need for approaches that stabilize the center shield while keeping the DTS between read sensors small.